1. Field of the Invention
The present invention relates to a vapor phase growth system and a gas-drive motor, and more particularly, to a vapor phase growth system in which rotating and revolving motions of wafers can be performed separately and independently of each other, a gas-drive motor particularly suited for use as a drive source for wafer rotation in the vapor phase growth system of this type, and a gas-drive motor system having a function to control the rotor rotating speed of the motor.
2. Description of the Related Art
In a conventional vapor phase growth method, a material gas is fed into the reactor along with a carrier gas while wafers held by a susceptor in a reactor are heated through the medium of the susceptor, to thereby carry out vapor phase growth of an epitaxial layer on the surface of each wafer. The epitaxial wafers, obtained by this vapor phase growth method, is subject to a post-treatment, whereby devices of electrical apparatuses are manufactured. In order to improve the yield in the post-treatment and the quality of the devices, differences in the thickness of the epitaxial layer formed on the wafer surface must be reduced.
To attain this, a known vapor phase growth system (Jpn. Pat. Appln. KOKOKU Publication No. 1-216521) is designed so that wafers are revolvable around the axis of a susceptor and rotatable on their own axes. Since the wafers rotate and revolve in a stream of a material gas, according to this apparatus, the material gas can be supplied uniformly to various parts of the wafer surface, so that the differences in the thickness of the epitaxial layer can be reduced.
As described in detail later with reference to FIG. 1, however, the aforesaid conventional vapor phase growth system is arranged so that each wafer rotates on its own axis as it revolves, and the ratio of the wafer rotating speed to the wafer revolving speed is fixed. Accordingly, even though the wafer rotating and revolving speeds are varied, their combination is restricted. Depending on the specifications of the epitaxial wafers, therefore, both the wafer rotating and revolving speeds cannot be adjusted to optimum values, so that the differences in the thickness of the epitaxial layer may increase. In order to change the ratio of the wafer rotating speed to the wafer revolving speed, moreover, gears and other relevant parts of a drive system for wafer rotation/revolution must be replaced, so that the manufacturing efficiency of the epitaxial wafers is lowered.
Since the susceptor is heated to increase the wafer temperature to a proper level, as described above, the surroundings of the susceptor are heated to a high temperature. In some cases, furthermore, the reactor may be decompressed in order to restrain auto-doping of the wafers. Thus, the vapor phase growth system may be operated at high temperature or under reduced pressure.
If the apparatus is operated at high temperature with use of a conventional electric motor as the drive source for wafer rotation/revolution, electrical supply to the motor may possibly cause an insulating coating film on ordinary insulated windings to be deteriorated or fuse. If the apparatus is operated under reduced pressure, moreover, the insulating film may produce impurity gases.
To cope with this, a gas-drive motor has been tried as a drive source for wafer rotation/revolution which requires no electrical supply and has no problems aroused by the use of insulated windings. The conventional gas-drive motor is provided with a rotor which is located in a stator and has a plurality of vanes, and is designed so that the rotor rotates when a gas is jetted against the vanes.
However, the gas-drive motor of the vane type is subject to some drawbacks. Since the rotor is generally supported in the stator by means of bearings, it undergoes vibration which is attributable to sliding friction with the bearings as it rotates, so that a smooth rotational output cannot be obtained. In order to increase the rotor output torque, moreover, the vane-type gas-drive motor is furnished with a speed reducer. If this motor is used under reduced pressure, therefore, lubricating oil which is applied to the reducer evaporates. Furthermore, output rotating speed control of the motor of this type is a hard task.